Einstein's theory of special relativity says that a moving clock, when compared to a stationary clock, runs slow. And general relativity, his theory of gravity, says that the weaker the gravitational field, the faster a clock in that field runs. These predictions were tested in 1971 by flying atomic clocks around the world. Einstein's relativity theories correctly explained what happened.

To learn about a more accurate update to this experiment 25 years later, see Time Flies. Click on the image to see a photo of the atomic clocks in the updated experiment.

The false-color image above shows jet contrails in the skies above the mid-Atlantic coast on 1/26/2001. Visit The Contrail Effect to find out how contrails can affect Earth's climate, and how this was investigated after 9/11.

The photo shows a Global Positioning System satellite. To find out how the system works, visit How does GPS work?. Be sure to see the video to understand how three or four different GPS satellites specify your position on Earth.

This famous fractal is the Mandelbrot set. Click to make the image larger, and look at the boundary between the black and blue--it is made up of the larger image, at smaller and smaller scales. Check out this Fractal Geometry page from IBM, especially the video (scroll down) that zooms in on the Mandelbrot set.

The image is named for Benoît B. Mandelbrot, who made it by graphing a set of complex numbers described by an equation. He coined the term "fractal" and popularized fractal research by showing its importance to other fields besides mathematics. Read more about his life and accomplishments at Benoît Mandelbrot, Novel Mathematician, Dies at 85.

In 1984, astronaut Bruce McCandless made this untethered spacewalk--the first ever, and one of only a few. He maneuvered with a "jet pack" strapped to his body as he orbited Earth at about 18,000 miles an hour. Click on the image to see McCandless at his maximum distance from the shuttle.

Whenever the jet pack's engines were off, McCandless was in free fall--the only force on him was Earth's gravity (neglecting air resistance and the gravitational attraction of the shuttle). To learn more, visit Footloose and this APOD page.

This image shows the "Lorentz attractor," a graph that represents the behavior of a simple model of Earth's weather. Weather is just one example of a chaotic system, in which seemingly random behavior does follow certain patterns.

The long red tubes are zirconium-alloy-clad fuel rods being fastened together into large bundles that will form the core of a nuclear reactor. Inside the zirconium cylinders are stacked pellets of uranium oxide, the reactor fuel.

To find out what happens to the zirconium cladding and the fuel rods in a "nuclear meltdown", visit Mechanics of a Meltdown Explained. The article explains the problems faced by the Fukushima power plant after the March 2011 earthquake in Japan.

On Earth, the heat produced by the candle expands the nearby gas, and it makes the gas more buoyant, so it rises and produces the tall flame. Up in the Space Shuttle, it's quite different, since the Shuttle, the air inside, and the candle are in free fall; everything falls around Earth together, so there is no up or down created by gravity. In space, the flame spreads out equally in all directions, distributing the heat into a far larger volume than on Earth, and producing the cool blue flame.

This X-ray image of 3C273 shows a jet of energy shooting out of the quasar's bright center, thought to be home to a supermassive black hole. If you look closely you can see a small thread connecting the center to the bright spots of the jet. Scientists have observed that matter from that small thread moves very fast, then appears to slow down in the luminous part of the jet, akin to a "cosmic traffic pile-up" of matter. For more details, see Chandra Observes Cosmic Traffic Pile-Up In Energetic Quasar Jet.